1. Technical Field
The present invention relates to surgical interference screws, especially those used in ligament reconstruction.
2. Discussion of the Prior Art
Interference screws are surgical screws that interfere or wedge between bones or other firm tissue. A typical use is in reconstructing the anterior cruciate ligament of the knee whereby a tunnel is drilled into the femur, and the ligament is inserted and secured inside the tunnel. The other end of the ligament is inserted similarly into the tibia. Procedures for reconstructing this ligament are described in U.S. Pat. No. 5,139,520, issued to Rosenberg on Aug. 18, 1992, and in U.S. Pat. No. 4,927,421, issued to Goble et al, on May 22, 1990. The disclosures from both of these patents are incorporated herein by reference.
The ligament (either graft or prosthetic ligaments can be used) may, for example, terminate in a bone block or bone plug fixed inside the tunnel by the interference screw. Alternatively, the ligament may terminate in a suitable polymer (or other firm material) block or plug serving as an attachment point for the ligament. The screw is driven into the gap between the tunnel wall and the bone plug to wedge the bone plug in place.
Interference screws often become skewed (i.e., turn aside from their intended direction) if they are started at the wrong angle or meet an obstacle. Also, the interference screw may disengage from its driver and become lost in the patient's body. When the screws are used in endoscopic operations, as is typical, these difficulties are multiplied, and the proper starting position may be difficult to locate as well.
Guide wires are often employed to prevent interference screws from being lost or mis-aligned. The wire is fed into the bone tunnel between the tunnel wall and bone plug. By means of an axial cannula extending the length of the interference screw, the screw is slid over the emplaced wire to bring the screw to the proper bone entry point and help keep the screw aligned with the intended path.
Interference screws usually form their own threads in the engaged bone tissue by cutting and removing material. To rotate the interference screw and force the threads into the bone, a special driver is used and includes a cannula with a transverse cross-section identical to that of the interference screw. The guide wire passes through both the screw and driver and allows the driver to engage the socket with the guide wire in place.
An interference screw usually has no screw head extending transversely beyond the shank radius in order to permit the entire screw to be driven into the bone tissue. Accordingly, the threads generally, but not necessarily, run all the way from the distal point to the rear of the screw, and no structure extends laterally beyond the minor screw thread diameter. Instead of a slot, cross, or Phillips head, a female drive socket, usually hexagonal, is provided at the rear end of the interference screw.
The diameter and strength of the guide wire are limited by the use of a female socket and a male driver which must contain the guide wire within. Given that the screw has a certain diameter and a certain thread height, the shank diameter (i.e., the minor thread diameter) is thereby fixed. Around the rear socket, a minimum wall thickness is needed for mechanical strength, so the outermost points of the socket define a first circle having a diameter equal to the shank diameter minus the wall thickness. The socket must be contoured to engage the driver; for example, it might be starred, lobed, squared, hexagonal, etc. The innermost points of the socket (i.e., those socket points of least radius) define a second circle of smaller diameter. The driver must be slightly smaller than the engaged socket, so the innermost points of the driver lie on a third circle of diameter smaller than that of the innermost points of the socket. The wall of the driver shaft, between the innermost driver points and the driver cannula, must be of substantial thickness if the driver is not to bend or buckle in use, so the cannula defines a fourth circle of diameter smaller still. The guide wire itself is smaller than the cannula of the driver for clearance. All these reductions mean that the guide wire diameter is severely limited in relation to the screw diameter which itself is generally only a few millimeters. The severely limited guide wire diameter lowers its stiffness and resistance to kinking. Specifically, wire strength varies with the square of its diameter, so that if the diameter is halved, the strength is quartered. Accordingly, the small diameter of interference screws places strict limits on guide wire strength.
Screws often skew when started and burrow into the bone in the wrong direction unless they are held in alignment while being driven. The guide wire is too weak to hold the interference screw properly; that is, the wire can bend or kink permanently under the strong lateral forces sometimes exerted during screw advancement. The guide wire can also kink when being fed into the bone tunnel. Engagement of the driver tip with the socket helps to align the screw, but the engagement length is typically so short that the driver tip cannot secure the interference screw alignment.
In addition to the risk of the screw turning from its path, guide wires have the additional problem of being hard to emplace. In view of the fact that the wires are flexible and can whip about, the surgeon cannot easily discern, from the orientation of the observed portion, the direction in which the wire tip is pointing.
A refinement of the guide wire method for keeping an interference screw aligned is described in the '421 Goble et al. patent, mentioned above. Goble et al discloses an interference screw having an integral drill bit at the forward end and a helical thread wound about only the rear portion of the screw. The bit has a diameter equal to the minor diameter of the thread. The guide wire, cannula, hexagonal socket, and driver are conventional. The Goble et al interference screw is held in alignment only by the bit in its bored hole, the guide wire, and the driver inserted in the screw socket. However, the engaged driver bit is short and can cut sideways as well as forwardly. Further, the Goble et al bit has tapered flutes for rearwardly directing cut bone chips between the threads in the region where the threads are exerting great force against the bone. If the interference screw is to penetrate far into bone, the cutting action may be decreased when the chips fill the flutes and jam under the cutting edge. The sharp cutting edge of the Goble et al bit can also irritate or tear soft tissues, such as ligaments and muscles, if the interference screw should migrate out of the bone or be emplaced wrongly.
A further refinement is marketed by Acumed, Inc. as the Oregon Fixation System. This system employs a fixation screw having an integral elongated guide member projecting forwardly from its distal end with a transverse dimension significantly smaller than the root diameter of the screw. The guide member is described in promotional literature as "assuring ease of use and accurate placement eliminating the need for guide pins". The proximal end of the screw is provided with a hexagonal socket for receiving a hexagonal driver. This system is an improvement over systems using guide wires but still does not have the inherent stability during insertion, nor the flexibility for screw revision, of the present invention as described hereinbelow.
Another drawback to conventional interference screws is that they cannot be revised easily. The screw may need to be revised if the ligament graft fails, the screw migrates, or some other problem develops after the surgery. A cannulated interference screw, if successfully emplaced by the use of a guide wire, is left in place after the wire is withdrawn. With the wire withdrawn, there is nothing to guide the driver back to the screw socket in order to turn the screw back out. Unguided location of the socket is difficult because the socket is a small target, the screw is surrounded by hard bone which feels like the screw when probing, and the blunt-tipped hexagonal driver must be used to probe instead of a pointed instrument that could more easily find the socket. Moreover, the bone tunnel begins to fill with new tissue, thereby imprisoning the screw, as time elapses after the original surgery. All of this makes revision difficult. The previously withdrawn guide wire that was used to guide the interference screw into the gap is useless during revision, since it is even more difficult to thread the guide wire into the screw cannula than to put the driver into the emplaced socket.
In reconstructing the anterior cruciate ligament of the knee, it may be impractical for the driver to approach the screw for revision from the originally driven end, depending upon the original screw placement and how much tissue growth has occurred in the tunnel since completion of the operation. It is desirable, therefore, for a surgeon to be able to approach the interference screw from either end in order to remove it. However, no prior art interference screw can accept a driver at the forward tip.
In sum, the guide wires and prior art interference screws do not present a satisfactory answer to the problem of interference screw angular stability when driving or to the problem of revising interference screws after emplacement.